Re: [PATCH v3 1/3] x86/hyperv: Use slow_virt_to_phys() in page transition hypervisor callback

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On Fri, Jan 12, 2024 at 07:24:35PM +0000, Michael Kelley wrote:
> From: Edgecombe, Rick P <rick.p.edgecombe@xxxxxxxxx> Sent: Friday, January 12, 2024 9:17 AM
> > 
> > On Fri, 2024-01-12 at 15:07 +0000, Michael Kelley wrote:
> > > The comment is Kirill Shutemov's suggestion based on comments in
> > > an earlier version of the patch series.  See [1].   The intent is to
> > > prevent
> > > some later revision to slow_virt_to_phys() from adding a check for
> > > the
> > > present bit and breaking the CoCo VM hypervisor callback.  Yes, the
> > > comment could get stale, but I'm not sure how else to call out the
> > > implicit dependency.  The idea of creating a private version of
> > > slow_virt_to_phys() for use only in the CoCo VM hypervisor callback
> > > is also discussed in the thread, but that seems worse overall.
> > 
> > Well, it's not a huge deal, but I would have just put a comment at the
> > caller like:
> > 
> > /*
> >  * Use slow_virt_to_phys() instead of vmalloc_to_page(), because it
> >  * returns the PFN even for NP PTEs.
> >  */
> 
> Yes, that comment is added in this patch.
> 
> > 
> > If someone is changing slow_virt_to_phys() they should check the
> > callers to make sure they won't break anything. They can see the
> > comment then and have an easy time.
> > 
> > An optional comment at slow_virt_to_phys() could explain how the
> > function works in regards to the present bit, but not include details
> > about CoCoO VM page transition's usage of the present bit. The proposed
> > comment text looks like something more appropriate for a commit log.
> 
> Kirill -- you originally asked for a comment in slow_virt_to_phys(). [1]
> Are you OK with Rick's proposal?

Yes, sounds sensible.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov




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